0 A.D. is a free and open-source real-time strategy (RTS) game of ancient warfare. In short, it is a historically-based war/economy game that allows players to relive or rewrite the history of Western civilizations, focusing on the years between 500 B.C. and 500 A.D.
The project is highly ambitious, involving state-of-the-art 3D graphics, detailed artwork, sound, and a flexible and powerful custom-built game engine.
0 A.D. is available by default in official repositories in Ubuntu 12.04. So if you haven't tried the game, give it a shot now as it is one of the best open source gaming projects.
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